Apparatus for measuring the mass flow rate of air are used for various applications, such as in fuel injection systems. Known apparatus typically include a probe and a bridge circuit, wherein the diagonal voltage of the bridge circuit is applied to a switching control element. The switching control element is in turn supplied with voltage pulses for heating the probe. The switching control element thus regulates the heat output conveyed to the probe. The apparatus emits an output signal proportional to the mass flow rate of air over the probe. A typical apparatus for measuring the mass flow rate of air is shown in DE-PS 24 48 304.
In known circuits for controlling apparatus for measuring the mass flow rate of air, or flow sensors, the heating current typically has a direct current component and an alternating current component. The alternating current component is superimposed on the direct current component and consists of a series of current pulses, each of constant duration, for heating the sensor's probe. The pulse repetition rate, which is the pulse frequency, changes depending upon the mass flow rate of air over the probe.
In a typical known circuit for controlling a flow sensor, an operational amplifier, whose output voltage is changed to a frequency proportional to the mass flow rate of air over the probe, is connected to resistance bridge diagonals. The frequency of the voltage controls the alternating current supplied to the bridge circuit and serves as a controlled variable for use in a fuel injection system. For this purpose, the output voltage of the operational amplifier is changed to a frequency by means of a voltage to frequency converter, and the operational amplifier's output voltage is subjected to a linearization function.
For many purposes, it is desirable to obtain an analog output voltage as the output signal of a switching controlled flow sensor. One problem with known circuits for controlling flow sensors, however, is that the output signal is dependent on the battery supply voltage. Thus, because the analog output voltage is dependent on the battery supply voltage, changes in the battery supply voltage can affect the level of the flow sensor's analog output signal.
It is the object of the present invention, therefore, to overcome the problems of known apparatus for measuring the mass flow rate of air.